Means and method for optimizing refined oil yield from a solvent refining unit

ABSTRACT

A CONTROL SYSTEM CONTROLS A SOLVENT REFINING UNIT WHICH REFINES CHARGE OIL TO YIELD REFINED OIL AND EXTRACT OIL USING A MIXTURE OF AT LEAST TWO SOLVENTS OF DIFFERENT ECONOMIC VALUES. A CONDITION, SUCH AS THE REFRACTIVE INDEX OR THE SPECIFIC GRAVITY, OF THE CHARGE OIL, OF THE REFINED OIL AND OF THE EXTRACT OIL IS SENSED AND CORRESPONDING SIGNALS CC, CR AND CE, RESPECTIVELY, ARE PROVIDED. A NETWORK PROVIDES A SIGNAL CORRESPONDING TO THE REFINED OIL VOLUME PERCENT YIELD Y IN ACCORDANCE WITH THE CONDITION SIGNALS CC, CR AND CE AND THE FOLLOWING EQUATION: Y = (CE-CC)/(CE-CR) THE REFINED OIL YIELD Y IS USED TO DETERMINED THE RATIO OF SOLVENT MIXTURE TO REFINED OIL TO CONTROL THE SOLVENT MIXTURE WHILE MAINTAINING THE QUALITY OF THE REFINED OIL. THE CONTROL SYSTEM CHANGES THE FLOW RATE OF ONE OF THE SOLVENTS IN A STEPPING FASHION TO AFFECT THE SOLVENT MIXTURE. THE SOLVENT MIXTURE TO REFINED OIL RATIO FOR A CURRENT STEP IS COMPARED WITH THE SOLVENT MIXTURE REFINED OIL RATIO OF THE NEXT PREVIOUS STEP. WHEN A MINIMUN SOLVENT MIXTURE TO REFINED OIL RATIO IS OBTAINED, THE CONTROL SYSTEM MAINTAINS THE SOLVENT MIXTURE AND THAT SOLVENT MIXTURE TO REFINED OIL RATIO.

A. sEQUElRA. JR 3,799,871 MEANS AND METHOD FOR OPTIMIZNG REFINED OIL March Z6, 1974 YIELD FROM A SOLVENT REFINING UNIT 4 sheets-sheet 1 Filed Dec. 30, 1971 March 26, 1974 A. SEQUElRA, JR 3,799,871 I l l MEANS AND METHOD FOR OPTIMIZING REFINEI) OIL YIELD FROM A SOLVENT REFINING UNIT Filed Dec. 30.1971 4 Sheets-Sheet 3 E lL "l E40 l 239') 240) 24|) 243) I TRANsEE l1-+av i AND R 5 STORAGE D/A 4| y l L, GATES REGISTER coNv. I@

l I 4| a l MoNosTABLE i L, J l l MULTIVIBRATOR FIG. 4A E25 l I FIG. 4B

FEG. 4C

FIG. 4D

FiG. 4E

FIG. 4F

March 26, 1974 A. SEQUEIRAJR 3,799,871'

MEANS AND METHOD FOR OPTIMIZING REFINED OIL YIELD FROM A SOLVENT REFINING UNIT Filed Dec. 50, 1971 4 Sheets-Sheet 4.

QUALITY CONTROL SYSTEM 205 @L lllmilvullwnilllll l l||ll|l||| 2 2 A llllo TT E B E 9 nmLLA 9 d ITMMUR I 3 S V United States Patent O MEANS AND METHOD FOR OPTIMIZING RE- FINED OIL YIELD FROM A SOLVENT REFIN- ING UNIT Avilino Sequeira, Jr., Nederland, Tex., assignor to Texaco Inc., New York, N.Y. Filed Dec. 30, 1971, Ser. No. 214,313 Int. Cl. Cg 21/00 U.S. Cl. 208-324 13 Claims ABSTRACT OF THE DISCLOSURE Y-m-m The refined oil yield Y is used to determine the ratio of solvent mixture to refined oil to control the solvent mixture while maintaining the quality of the refined oil. The control system changes the flow rate of one of the solvents in a stepping fashion to affect the solvent mixture. The solvent mixture to refined oil ratio for a current step is compared with the solvent mixture refined oil ratio of the next previous step. When a minimum solvent mixture to refined oil ratio is obtained, the control system maintains the solvent mixture and that solvent mixture to refined oil ratio.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to control systems in general and, more particularly, to a control System for a solvent refining unit.

BACKGROUND OF THE PRIOR ART Heretofore, control systems for the solvent refining units, controlled the charge oil flow rate and the extractmix temperature. Control systems of this type are disclosed in U.S. patent applications Ser. No. 98,193, now Pat. No. 3,666,931 and Ser. No. 136,003, now Pat. No. 3,718,809, which are assigned to Texaco, Inc., assignee of the present invention. However, as pointed out in U.S. application Ser. No. 102,344, now Pat. No. 3,686,488, assigned to Texaco, Inc., it may be desirable to operate a solvent refining unit for maximum production within the units constraint limits even though the operating condition is not an optimum operating condition.

The control system of the present invention controls a solvent refining unit which uses a solvent mixture of at least two solvents of different economic values. It controls the solvent mixture to obtain a desired quality refined oil yield with the most economical solvent mixture. Since the solvent mixture and the solvent mixture to oil ratio is being controlled, the system of the present invention may be used in conjunction with the control system disclosed in the aforementioned U.S. application Ser. No. 102,344 to provide an optimum condition for maximum production. Furthermore, the control system of the present invention may be operated independently of the maximum production control system.

3,799,871 Patented Mar. 26, 1974l SUMMARY OF THE INVENTION A system controls a solvent refining unit which uses a solvent mixture of at least two solvents having different economic values when refining charge oil to yield refined oil and extract oil. The system includes a source which provides a signal corresponding to the ratio of the solvent mixture to the refined oil. Apparatus controls the ow rate of one of the solvents to affect the solvent mixture in accordance with the ratio signal from the source to obtain the most economical solvent mixture for a minimum solvent mixture to refined oil ratio.

The objects and advantages of the present invention will appear more fully hereinafter from a consideration 0f the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the present invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWINGS FIG. l includes a simplified block diagram of apparatus, constructed in accordance with the present invention, for controlling a solvent refining unit, which is also shown in schematic form, to use the most economical solvent mixture maintaining the quality of the refined oil.

FIG. 2 is a detailed block diagram of the solvent mixture control system shown in FIG. 1.

FIG. 3 is a detailed block diagram of one of the sample and hold circuits shown in FIG. 2.

FIGS. 4A through 4F are diagrammatic representations of pulse voltages occurring during the operation of the solvent mixture control system shown in FIGS. l and 2.

FIG. 5 is a detailed block diagram of the quality control system shown in FIG. 1.

DESCRIPTION OF THE INVENTION FIG. 1 depicts a solvent refining unit of the type disclosed in detail in U.S. Pat. 3,476,681. Elements having numbers 1 through 73 and 110 are the same as in the aforementioned patent, except that a portion of the water solvent, hereinafter referred to as water, being discharged through line 59 is now provided to the N-methyl2pyrrolidone solvent in line 3 as hereinafter explained so that line 3 carries a solvent mixture of two solvents having different economic values. The water has a lesser economic value than the N-methyl-Z-pyrrolidone. As a brief resume of the U.S. patent, charge oil entering extractor 2 by Way of line 1 is countercurrent contacted with the solvent mixture from line 3 to provide raffinate in line 5 and extract-mix in line 4. The raiiinate is processed to yield refined oil in line 28 while the extract mix is processed to yield extract oil in line 65. The solvent mixture removed from the rafiinate and extract mix during processing is returned by way of line 3 to extractor 2.

A solvent mixture control system 200 cooperating with a quality control system 205 controls the solvent mixture in line 3 and the ratio of solvent to refined oil to provide the most economical solvent mixture while maintaining the quality of the refined oil. Control system 200 controls the solvent mixture in line 3 as a function of a solvent mixture to refined oil ratio. Control system 205 controls the quality of the refined oil at a predetermined value or higher by maintaining the refractive index of the refined oil within predetermined limits.

Refractive index meters 201, 202 and 203 receiving representative samples of charge oil from line 1, refined oil from line 28 and extract oil from line 65, respectively, provide signals E1, E2 and E3, corresponding to the refractive indices of the charge oil, the refined oil and the extract oil, respectively, to control system 200. Control system 200 receives direct current voltages V1 and V2. Meters 201, 202 and 203 may dispose of the received oils as slop. or may return the oils to their respective lines. A ow sensor 207, which may be of a conventional type, senses the flow rate of water in a line 208, and provides a corresponding signal E23, as the water passes from line 59 to line 3 where it mixes with the N-methyl-'Z- pyrrolidone from distillation tower 68. Control system 200 provides a signal E24 to a valve 210 to adjust valve 210 thereby controlling the quantity of water entering line 3 to control the solvent mixture in line 3. Control system 200 cooperating with control system 205 increases the water flow rate and solvent mixture flow rate in a stepping fashion until a minimum solvent mixture to refined oil ratio within predetermined quality limits is obtained. Control system 205 also controls the flow rate of the charge oil in line 1 to affect the quality of the refined oil.

Control system 205 provides signals E62, E63 to control the position of a set point in a conventional flow recorder controller .104. Flow recorder controller .104 operates a valve 112 in accordance with the difference between the sensed charge oil -fiow rate in line 1, represented by a signal E37 from a conventional ow rate sen sor 105 and a target flow rate represented by the position of the set point, to control the flow rate of the charge oil.

Referring to FIGS. 1 and 2, a switch 214 receiving voltage V1 controls the operation of control system 200. Switch 214 is momentarily activated to momentarily pass voltage V1 thereby triggering a flip-flop 215 to a set state. A flip-flop provides a high level direct current output while in a set state and a low level direct current output while in a clear state.

The high level output .from flip-flop 215 enables an AND gate 216 so that AND gate 216 passes timing pulses E26, as shown in FIG. 4A from `a clock 217. The pulse repetition rate of pulses E25 is selected so that the solvent refining unit shown in FIG. 1 will stabilize after each pulse E26 and before the next pulse E25. Each passed pulse from AND gate 216 triggers a monostable multivibrator 218 to provide a pulse E26, as shown in FIG. 4B, to another AND gate 219. A direct current signal E27 from control system 205 is inverted by an inverter 220 so that when signal E27 is at a low level, inverter 220 provides a high level output to partially enable AND gate 219 along with a high level signal E26. Each pulse E26 from multivibrator 218 fully enables AND gate 219 to pass pulses E29, Shown in FIG. 4C, from a clock 221. The width of pulse E26 is selected to allow a predetermined number of pulses E22 to pass which correspond to a predetermined change in the Water flow rate. By way of example, FIGS. 4B, 4C and 4D show three pulses E29 being passed for each pulse E26. The passed pulses E29 from AND gate 219 are applied to a flow recorder controller 222, through an OR gate 223 as pulses E26, which are shown in FIG. 4D, and an AND gate 224, which provides pulses E25, to change the set point of controller 222 in a direction in accordance with a direct current directional signal E31 which is developed as hereinafter explained, AND gate 224 controls the change of controller 222 set point as a function of the quality of the refined oil as hereinafter explained.

Flow recorder controller 222 controls the quantity of water being mixed with the N-methyl-2-pyrrolidone in line 3 by controlling the water flow rate in line 208 in accordance with directional signal E22 and pulses E36. When signal E21 is at a high level, a change may be made in controller 222 set point position which increases the water flow rate as hereinafter explained. When signal E31 is at a low level, controller 222 set point may be changed to reduce the water flow rate. Flow recorder controller 222 receives signal E22 from sensor 207 corresponding to the water fiow rate in line 208 and provides signal E24 to valve 21o when the sensed new rete differs from the target flow rate as determined by the position of controllers 222 set point. Signal E24 operates valve 210 so that the water flow rate in line 208 is substantially equal to the target flow rate.

The trailing edge of pulse E26 from multivibrator 218 triggers a clock 238. Clock 238 provides a sampling pulse E34 shown in FIG. 4E, after a time delay, to another monostable multivibrator 218A. The time delay provided by clock 238 should allow sufficient time for the solvent refining unit to reach a steady state after the change resulting from an E26 pulse. The trailing edge of the sampling pulse E24 from clock 238 triggers a monostable multivibrator 218A causing it to provide a sampling pulse E36, as shown in FIG. 4F. Sampling pulses E24 and E26 are used to control sample and hold circuits, as hereinafter explained, so that a current solvent mixture to refined oil ratio may be compared with the next previous solvent mixture to refined oil ratio. Elements having a number with a suffix are connected and operate in a similar manner to elements having the same number without a suffix.

Flow recorder controller 222 continues to increase the water flow rate in a stepping fashion in response to pulses E26 until the solvent mixture to refined oil ratio for a current step is greater than the solvent mixture to refined oil ratio for the next previous step. The equation for the refined oil volume percent Y is as follows:

where Cc, CR and CE are conditions of the charge oil, of the refined oil and of the extract oil, respectively. The conditions may be the refractive indices or specific gravities of the oils. Where the conditions are the refractive indices, Equation 1 may be rewritten as (2) RIE-RIC RIE-R13 where RIC, RIR and RIE are the refractive indices of the charge oil, the refined oil and the extract oil, respectively. In determining and comparing the solvent mixture to refined oil ratios, subtracting means 227, 228 subtract signals E1 and E2, respectively, from signal E2 to provide signals corresponding to the terms RIE-RIC and RIE-RIE, respectively, of the equation. A divider 229 divides the signal from subtracting means 227 with the signal from subtracting means 228 to provide a signal corresponding to the refined oil yield Y. The refined oil yield signal is multiplied with a signal E37, from a sensor A in line 1, corresponding to the charge oil flow rate, by a multiplier 231. The signal .from multiplier 231 corresponds to the flow rate of the refined oil. Due to the time lags of the elements in producing refined oil, it is preferred to calculate the refined oil flow rate rather than measure it directly.

Signal E4 from sensor 105C, which corresponds to the flow rate of the solvent mixture in line 3, is divided by the signal from multiplier 231 by a divider 236 to provide a signal E46 corresponding to the solvent mixture to refined oil ratio.

Signal E46 is applied to a sample and hold circuit 237 which is controlled by sampling pulse E26 from multivibrator 218A. Sample hold circuit 237 may be of the type shown in FIG. 3 in which signal E46 is converted to digital signals, by a conventional type analog-to-digital converter 239, that are applied to transfer AND gates 240. And gates 240 represent a plurality of AND gates, each AND gate receives a different digital signal from converter 239. Sampling pulse E26 is applied to a storage register 241, which may be of a conventional type, and to a monostable multivibrator 242. Register 241 is reset by a pulse E32 while multivibrator 242 is triggered by the trailing edge of pulse E36. When triggered, multivibrator 242 provides a transfer pulse to each AND gate of AND gates 240 causing each AND gate to pass its received digital signal to register 241. Register 241 holds the digital signals until reset by another pulse E35. Digital signals from register 241 are converted to an analog signal, by a conventional digital-to-analog converter 243, which is provided as a signal E41 from sample and hold circuit 237 corresponding to the current solvent to refined oil ratio. Signal E41 is applied to another sample and hold circuit 237A and to a comparator 245.

'Sample and hold circuit 237A is controlled by sampling pulse E34 to sample and hold signal E41 to provide a signal 'E42 to comparator 245. Signal E42 corresponds to the solvent to refined oil ratio for the next previous step. Comparator 245 compares the current solvent mixture to refined oil ratio signal E41 with the next previous solvent mixture to refined oil ratio signal E42. Comparator 245 provides a direct current signal E43 to an electronic switch 246 at a high level when the current ratio is equal to or less than the next previous ratio and at a low level when the current ratio is greater than the next previous ratio.

Electronic switch 246 is similar in operation to a single pole double throw switch and is controlled by signal E27 from control system 205. When signal E27 is at a high level, switch 246 passes a direct current signal E43 as directional signal E31 and blocks direct current signal E27. When signal E27 is at a low level, switch 246 blocks signal E43 and passes signal E22 as signal E31.

Since, it is not known that a minimum solvent mixture to refined oil ratio is reached until the ratio for current operating step is greater than the ratio of the next previous operating step; the solvent refining unit must be returned to the operating conditions of the next previous step so as to achieve the minimum ratio.

When signal E42 changes to a low level, as a result of the current ratio being greater than the next previous ratio, the negative going signal E42 is inverted by an inverter 247. The positive going output from inverter 247 passes through an OR gate 248 and is inverted by an inverter 249 to a ne-gative going voltage. The negative going voltage toggles a flip-flop 250 to a set state. Flipflop 250 was placed in a clear state by voltage V1 momentarily passed by switch 214. The high level output .from flip-fiop 250 enables clock means 251 and passes through an OR gate 252 to enable AND gate 224. Clock means 251 provides pulses at a repetition rate which is sufficient to allow one more pulse E25 to be passed by AND gate 216.

The last pulse E25 passed by AND gate 216 causes multivibrator 218 to provide a pulse E25 which results in AND gate 219 passing some pulses E29 as heretofore described. Since AND gate 224 was still enabled by the high level output from ip-op 250, the resulting pulses E30 pass through AND gate 224. However, since E43 is at a loW level, directional signal E31 is at a low level and the set point of flow recorder controller 222 is changed by pulses E35 to decrease the water flow rate in line 208. The solvent refining unit is thus returned to the next previous step which provided the maximum refined oil yield for a minimum solvent to refined oil ratio. Since this was achieved with the addition of water to the solvent mixture, the solvent mixture is more economical than prior to the control.

Clock means 251 provides a pulse after the next E25 pulse which triggers fiip-ops 215, 250 to a clear state. The pulse from clock means 251 pased through OR gate 248 and was inverted by inverter 249 to toggle flipflop 250 to the clear state to disable clock means 251.

Referring now to FIGS. l, 2 and 5, when the water from line 208 is added to the solvent in line 3, the yield of the refined oil will increase; however, eventually the quality of the refined oil will decrease. Control system 205, receiving direct current voltages V4 through V6, and signals E2, E23, maintains the quality of the refined oil by maintaining the refractive index of the refined oil within predetermined values. In controlling the refractive index of the rened oil, control system 205 will increase the charge oil flow rate in line 1 when the refined oil refractive index is below a predetermined lower limit and water is not being added to the solvent mixture in line 3; and increase the charge oil flow rate when the refractive index is greater than a predetermined upper limit and the refined oil refractive index for the current step is less than the refined oil refractive index for the next previous step. Furthermore, when the refined oil refractive index is less than the lower limit and water is being added to the solvent-mixture in line 3, the control system 205 will increase the water flow rate in line 208.

Comparators 301, 302 receive signal E2 from refractive index meter 202 and voltages V4 and V5 respectively. Voltages V4, V5 correspond to the upper and the lower limits, respectively, for the refined oil refractive index. When the refined oil refractive index is within the two limits or equal to one of the limits, signal E2 is equal to voltage V4 or V5, or less positive than voltage V4 and more positive than V5, which causes comparators 301, 302 to provide low level direct current outputs. Inverters 304, 305 invert the low level outputs from comparators 30.1 and 302, respectively, to high levels. An AND gate 306 is responsive to the high level outputs from inverters 304, 305 to provide direct current signal E23 at a high level to partially enable AND gate 219, thereby permitting control system 200 to control the water ow rate in line 208.

When the refined oil refractive index is outside of the limits, one of the comparators 301 or 302 provides a high level output which is inverted to a low level by a corresponding inverter 304 or 305, respectively. AND gate 306 is responsive to the low level output from in- Iverter 304 or 305 to provide signal E23 at a low level. When signal E25 goes to a low level, AND gate 219 is disabled and does not pass any pulses E23 so that the refining unit is effectively controlled by quality control system 205 as hereinafter explained.

For the condition where the refined oil refractive index is greater than the predetermined upper limit, comparator 301 provides a high level output which enables AND gates 3114, 316. A sample and hold circuit 237B is controlled by sampling pulse E35 to sample and hold signal E2 to provide an output corresponding to the refined oil refractive index for the current operating step. A sample and hold circuit 237C is controlled by sampling pulse E24 to sample and hold the output from the sample and hold circuit 237B to provide an output corresponding to the refined oil refractive index for the next previous operating step. A comparator 245A compares the outputs from circuits 237-B, 237C and provides a high level direct current output to AND gates 314, 316 when the output from circuit 237B is less than circuit 237C output anda low level output when circuit 237B is equal to or greater than circuit 237C output. AND gate 3'14 provides a high level output in response to the high outputs from comparators 245A, 301. The high level output is applied to a clear input of a Hip-flop 31'5 causing iiip-ffop 315 to change to its clear state. While in the clear state, flip-Hop 315 provides a low level output as a directional signal E52 to Iflow recorder controller 104.

Comparators 301, 245A high level outputs also enable AND gate 316 to pass pulses E254 from a clock 217A to another AND gate 219A through an OR gate 320. The positive going change in the output from AND gate 314 passes through an OR gate 321 and is inverted by an inverter 325. The negative going output from inverter 325 triggers a monostable multivibrator 218A. Multivibrator 218A cooperates with AND gate 219A to pass a certain number of pulses from AND gate 316 as pulses E53, corresponding to a predetermined change in the charge oil flow rate to controller 104. Since signal E52 is at a low level, the change results in a decrease in the charge oil ow rate.

When the oil refractive index is less than the lower limit, signal E2 is less positive than voltages V4, V5 causing comparators 301 and 302, respectively, to provide a low level and a high level output respectively. Comparator 301 output disables AND gates 314, 316, thereby preventing sample and hold circuits 237B, 237C and comparator 245A from controlling the refining unit. The high level output from comparator 302 enables gates 329, 330. A comparator 331 compares signal E23 corresponding to the water flow rate in line 208 with voltage V6 which correspond to a substantially zero iiow rate. When water is not being added to line 3 from line 208, signal E2 is more negative or equal to voltage V6 causing comparator 331 to provide a high level direct current output to fully enable AND gate 329.

When fully enabled AND gate 329 provides a high level output to a set input of flip-flop 315 causing flip-flop 315 to change to a set state. While in the set state liipflop 315 provides a high level output as signal E52. AND gate 330 is also fully enabled by the high level outputs from comparators 302, 331 to pass pulses EA from clock 217A. When AND gate 329 output went from a low level to a high level, the output passed through OR gate 321 and was inverted by inverter 325. The negative going outp11-t from inverter 325 triggered multivibrator 218B, as heretofore explained, to provide an enabling pulse to AND gate 219A to permit a quantity of the pulses from OR gate 320 to pass through to be provided as signal 'E53 to flow recorder controller 104 to change the set point. Since signal E52 is at a high level, signal E53 changes the set point of flow recorder controller 104 in a direction to increase the ow rate of the charge oil in line 1.

When the refined oil refractive index is below the lower limit and water is being added to the solvent mixture in line 3, control system 205 controls the refining unit. Under such a condition it is desirable to increase the amount of water entering line 3. Since water is being added to the mixture in line 3, comparator 331 provides a low level output disabling AND gates 329, 330. Comparator 301 provides a low output lwhich disables AND gates 314, 316. Since AND gates 314, 316, 329 and 330 are disabled, the charge oil iiow rate cannot be changed. An inverter 335 inverts the low output from comparator 331 to a high level thereby enabling an AND gate 336 along with the high level output from comparator 302 causing AND gate 336 to provide directional signal E2, at a high level. A high level signal E27 permits control system 205 to control the refining unit by disabling AND gate 219 and controlling switch 246 to provide the high level signal E27 as the directional signal E31. Since signal E31 is now at a high level, when a change is made to the set point of flow recorder controller 222, it will be in a direction to increase the water iiow rate in line 208. An AND gate 216A is enabled by the high level signal E21 to pass pulses E253 from a clock 217B. Clocks 217B and 221A, A=ND gate 216A and 219B, and a monostable multivibrator 218C operate in the identical manner as clocks 217 and 221, AND gates 216 and 219, and multivibrator 218 to provide pulses E61 to flow recorder controller 222 in control system 200 through OR gate 223 and AND gate 224.

When the flow rate of the water in line 208 is changed, the iiow rate of the solvent mixture in line 3 must be changed accordingly. This is accomplished by applying signals E31 and E35 to another flow recorder controller 222A ywhich receives signal E., from fiow rate sensor 105C corresponding to the solvent mixture iiow rate. In receiving signals E31 and E35, the set point of liow recorder controller 222A follows the positioning of the set point of flow recorder controller 222 so that for any change made in the iiow rate of the water in line 208 there is a corresponding change in the fiow rate of the solvent mixture in line 3. Flow recorder controller 222 provides a signal E to a valve 300 in line 3 for controlling the valve to regulate the solvent mixture iiow rate. Signal E80 corresponds to the difference between the sensed solvent mixture ow rate represented by signal E4 and the desired solvent mixture flow -rate represented by the position of the set point in fiow recorder controller 222A. The valve 300 is controlled by signal E80 to regulate the ow rate of the solvent mixture so that it is substantially the same as the desired ow rate for the solvent mixture.

Although the foregoing description applies to the N- methyl-Z-pyrrolidone refining process, it is also applicable to various other extraction processes which use two or more solvents to control solvent power or selectivity. Processes of the above type include the phenol refining process, dimethylsulfoxide extraction, duo-sol extraction and sulfolane extraction.

The system of the present invention controls the solvent mixture in a solvent refining unit to obtain the most economical solvent mixture by providing a minimum solvent mixture to refined oil ratio while controlling the quality of the refined oil. The solvent mixture is changed in steps and a solvent mixture to refined oil ratio for a current step is compared with the solvent mixture to refined oil ratio for the next previous step until a minimum solvent mixture to refined oil ratio is obtained. The system of the present invention maintains the most economical solvent mixture.

The present invention may be used Iwith other control systems such as the type disclosed in U.S. Pat. No. 3,686,488, issued on Aug. 22, 1972 to Mr. R. A. Woodle and assigned to Texaco Inc., assignee of the present invention. The Woodle application discloses a control system for operating a solvent refining unit for maximum production of refined oil or extract oil. Essentially, the Woodle control system controls the charge oil ow rate in line 1 and the temperature of the extract-mix leaving extractor 2. Since the charge oil liow rate is also being controlled by system 205, a logic circuit would be used to permit control by control system 205, and the Woodle control system. The ow rates of the refined oil in line 28, of the extract oil in line 65 and of the extract mix leaving extractor 2 would also be sensed and corresponding signals provided to the Woodle control system.

What is claimed is:

1. An improved control system for a solvent refining unit which treats charge oil with a solvent mixture of at least two solvents having different economic values in a refining tower to yield ratiinate and extract-mix, strippers separate the solvent mixture from the raiiinate and from the extract-mix to provide refined waxy oil and extract oil, respectively, the solvent mixture is returned to the tower and the refined waxy oil is subsequently dcwaxed to provide refined oil, and in which the control system comprises means for controlling the operation of the refining unit, first means connected to the control means for providing control signals to the control means to operate the refining unit for a predetermined time period at a predetermined solvent mixture dosage-temperature combination so as to provide refined oil of a desired quality, means for measuring at least one condition of the extract oil and one condition of the refined waxy oil and providing signals corresponding thereto, means for measuring at least one property of the charge oil and providing a corresponding signal, signal means for providing signals corresponding to limitations of operating parameters of the reining unit and the refining operation, means connected to the condition measuringmeans, to the property measuring means and to the limitation signal means for determining which operating parameter is limiting and providing signals corresponding thereto, and second means connected to the determining means and to the control means for providing control signals to the control means after the predetermined time period in accordance with the signals from the determining means to control the operation of the refining unit so that the refining unit operates at a maximum capability while maintaining the quality of the refined oil; and wherein the improvement comprises means for providing a signal corresponding to the ratio of the solvent mixture to the refined oil, means for providing signals corresponding to the economic values of the solvents comprising the solvent mixture, and means connected to the ratio signal means to the value signal means for the controlling of the flow rate of one of the solvents to effect the solvent mixture in accordance with the ratio signal and the economic value signals to obtain the most economical solvent mixture for a minimum solvent to refined oil ratio While operating the refining unit at a maximum capacity.

2. A system as described in claim 1 in which the ratio signal means includes -means for providing a signal corresponding to the refined oil flow rate, means for sensing the flow rate of the solvent mixture and providing a signal representative thereof, and means connected to the flow rate sensing means and to the flow rate signal means for dividing the solvent mixture iiow rate signal with the refined oil flow rate signal to provide the ratio signal.

3. A system as described in claim 2 which further comprises means for sensing a condition of the charge oil, of the refined oil and of the extract oil and providing signals representative thereof, means connected to the condition signal means for providing a signal corresponding to the yield Y of refined oil in accordance 4with the condition signals and the following equation:

C'E-C'C Y CE CR where CE, CC and `CR are the sensed conditions of the extract oil, the charge oil and the rened oil, respective- 1y; and means for sensing the flow rate of the charge oil and providing a signal representative thereof; and in which the refined oil fiow rate signal means is connected to the refined oil yield signal means and to the charge oil flow rate sensing means and provides the refined oil ow rate signal in accordance with the refined oil yield signal andthe charge oil ow rate signal.

4. A system as described in claim 3 further comprising means for sensing the flow rate of one of the solvents and providing a signal corresponding thereto, means connected to the condition sensing means and to the solvent flow rate sensing means for providing a signal corresponding to the quality of the refined oil, and means connected to the quality signal means for controlling the flow rate of the charge oil in accordance with the quality signal to affect the quality of the charge oil. and in which the solvent flow rate control means is connected to the quality signal means and controls the flow rate of the one solvent so as to obtain the most economical solvent mixture for a minimum solvent mixture to refined oil ratio while controlling the quality of the refined oil.

5. A system as described in claim 4 which the sensed conditions are the sensed refractive indices of the charge oil, of the refined oil and of the extract oil; and the quality signal means includes means connected to the condition sensing means and receiving two direct current voltages corresponding to an upper limit and a lower limit, respectively, for the refined oil refractive index for determining relationship of the sensed refined oil refractive index to the limits and providing signals corresponding to the determination control means as the quality signal.

6. A system as described in claim 2 in which the water fiow rate control means includes step change means connected to the ratio signal means for changing the water fiow rate in a stepping fashion in accordance with the ratio signal s as to obtain a minimum solvent mixture to refined oil ratio.

7. A system as described in claim 6 in which the step change means includes means connected to the ratio signal means for providing a next previous ratio signal Cit and a current ratio signal corresponding to the ratio signal for the current step change and for the next previous step change, respectively, means for comparing the current ratio signal with the next previous ratio signal and providing an output of one amplitude when the current ratio signal is equal to 0r less than the next previous ratio signal and of another amplitude when the current ratio signal is greater than the next -previous ratio signal, rneans connected to the comparing means for periodically mcreasing the water solvent fiow rate by a predetermined amount when the output from the comparing means is of the one amplitude and decreasing the water ow rate for one step change by the` predetermined amount when the output from the comparing means changes from the one amplitude to the other amplitude.

8. A system as described in claim 7 further comprismg means for sensing the refractive index of the refined oil and providing a signal corresponding thereto, and in which the quality signal means includes means connected to the refractive index sensing means and receiving two direct current voltages corresponding to an upper limit and a lower limit, respectively for the refined oil refractive index for determining relationship of the sensed refined oil refractive index to the limits and providing a signal corresponding to the determination to the control means as the quality signal.

9. An improved method for controlling a solvent refining unit in which charge oil is treated with a solvent mixture of at least two solvents having different economic values in a refining tower to yield raffinate and extractmix, strippers separate the solvent mixture from the raffinate and extract mix to provide refined waxy oil and extract oil, respectively, the solvent mixture is returned to the tower and the refined waxy oil is subsequently dewaxed to provide refined oil, `which comprises refining the charge oil vfor a predetermined time period and a predetermined solvent mixture dosage and a predetermined temperature to achieve a desired quality of refined oil, measuring at least one property of the charge oil, measuring one condition of the refined waxy oil and one condition of the extract oil, providing signals corresponding to the measurements, providing signals corresponding to limitations of the refining unit and the refining operation, utilizing the signals to determine which operating parameter is limiting, and controlling the operation 0f the refining unit after the predetermined time period in accordance with the determination; and wherein the improvement comprises providing a signal corresponding to the ratio of the solvent mixture to the refined oil, providing signals corresponding to economic values of the solvents and controlling the fiow rate of at least one of the solvents to effect the solvent mixture in accordance with the ratio signal and the economic signals to obtain the most economical solvent mixture for a minimum solvent refined oil ratio while operatingthe refining unit at a maximum capacity.

10. A method as described in claim 9 which further comprises sensing a condition of the charge oil, of refined oil and of the extract oil, providing condition signals corresponding to the sensed conditions, providing a signal corresponding to the yield Y of refined oil in accordance with the condition signals and the following equation:

CE- CC where CE, Cc and CR are the sensed conditions of the extract oil, the charge oil and the refined oil, respectively; sensing the fiow rate of the charge oil, and providing a signal representative of the sensed charge ow rate; and in which the refined Oil fiow rate signal is provided in accordance `with the refined oil yield signal and the sensed charge oil flow rate.

11. A method as described in claim 10 where the sensed condition is the refractive index.

12. A method as described in claim 10 in which the sensed condition is the specific gravity.

13. A method as described in claim 12 in which the controlling step includes sampling and holding the ratio signal, changing the Iilovv rate of one of the solvents, in one direction by a predetermined amount, sampling and holding the ratio signal at a predetermined time after the one solvent flow rate change, comparing the samples to determine Whether the solvent mixture to refined oil ratio has increased, decreased or remained the same after the change in the flow rate of the one solvent, providing a control signal corresponding to the determination, repeating the changing step and the subsequent sample and holding step the comparing step and control providing step when the control signal corresponds to a determination that the solvent mixture to rened oil ratio has decreased remained the same, changing the one solvent ow rate in an opposite direction by the predetermined amount when the control signal corresponds to an increased solvent mixture to rened oil ratio, and maintaining the one solvent ilow rate after the one solvent ow rate has been changed in the opposite direction.

References Cited UNITED STATES PATENTS 3,539,784 11/1970 Woodle 23S-151.12 3,458,432 7/ 1969 Woodle et al. 208-36 3,476.681 11/ 1969 Davies et al. 208-326 3,470,089 9/ 1969 Morris et al. 208-324 3,451,925 6/ 1969 Morris et al. 20S-324 3,285,846 11/1966 King et al 208-28 3,666,931 5/ 1972 Woodle 208-311 3,686,488 8/1972 Woodle 23-151.12

HERBERT LEVINE, lPrimary Examiner U.S. Cl. X.R.

196-l4,52; 20S-DIG. 1; 23S-251.13 

